It is known to couple a plurality of interface units, such as power switching units to a microcomputer which controls the switching of the units via respective control lines.
Each such unit is usually provided with a fault indicating output which provides a fault indicating output signal in the event of a fault occurring in the unit. It is common to couple all the fault indicating output terminals together in a wired OR configuration and to the interrupt input of the microcomputer unit.
In the event of a fault occurring in one of the interface units an output signal at its fault indicating output will interrupt the microcomputer to alert it to the fault. In view of the wired OR configuration, it has hitherto not been possible to identify the unit originating the fault indicating signal without separate diagnostic procedures requiring the provision of an extra diagnostic pin and the disturbing of the output states of the units.